An Introduction to Zero-Point Energy

CalPhysics.org ^

[El Gato]

Just so everybody understands, it's the "push against" that you are (rightly) objecting to. The propeller or the jet don't "push against" the air, rather they move the air backwards and, by conservation of momentum, that causes the engine to move forwards. More properly it causes a force in the foward direction. The same as the recoil of a gun, which would "kick" just as much (well close enough) in a vacumn as it does down here at the bottom of the atmosphere.

[spunkets]

Planck frequency = 10⁴³ Hz. Article had 1043.

[Centurion2000]

TANSTAAFL

[RightWhale]

Cavorite is the way to go. Only thing is: be sure you have prepared a way to shut the motor down before you try to start it up. The first demonstration came close to disaster.

[spunkets]

Re: propellers

"they move the air backwards"

The English word push, describes that action perfectly.

[PatrickHenry]

unless somebody repeals the Second Law of Thermo.

Write your congressman.

[sourcery]

Original statement at issue:

The propellor or the jet engine of an aircraft push air backwards to propel the aircraft forward. A ship or boat propellor does the same thing with water. On Earth there is always air or water available to push against.

You challenged the accuracy of the statement with respect to the actual physics of propellor-driven locomotion. As I read the statement to which you object, it simply states that propellor-driven locomation requires a medium in which the propellor can operate, and mentions that the principle of operation involves "pushing air backwards." The statment is incomplete, but is not incorrect (a propellor does in fact blow air in the reverse direction of the airplane, as anyone who stands behind the propellor can attest.) Of course, it would be incorrect to imagine that forward thrust derives in any way from blowing air against the air behind the plane (which is apparently how you interpret the statement at issue.)

Forward thrust results partially from conservation of momentum: by throwing air molecules backward, the propellor experiences a motive force in the opposite direction, as a consequence of Newton's Third Law of Motion. To me, "pushing air backward" is an acceptable way to state this, when one trusts that the audience understands the physics of motion. Although the pressure gradient created by the action of the propellor also contributes to the thrust vector, failure to mention this component of the thrust is not a sin in this context.

[Physicist]

Go check your physics for "On Earth there is always air or water available to push against."

I'm afraid I don't understand what you're driving at, either. Could you spell it out for us?

[sourcery]

It's useless unless somebody repeals the Second Law of Thermo.

That would be my first impression. But I am humble, and am always willing to consider the possibility that I am wrong.

[mcsparkie]

I think that what they want to hear you say is that aircraft, ships, etc do not get "pushed" through the air, they get "pulled" through the air by the vacuum created forward of the proplusion

The propellor blades are slanted so that there is high pressure on the back of the blade and low pressure on the front. The difference, or net total force produces a thrust forward on the propellor. The vacuum on the front is limited to zero pressure, which is only 14 psi less than atmospheric. I would guess that the pressure increase on the back is more substantial.

[sourcery]

Planck frequency = 1043 Hz. Article had 1043.

Could be an error introduced when I copied from the browser page (which was rendered from HTML) into the posting form using simple ASCII.

[spunkets]

Yep. See 51

[Lurking Libertarian]

evolution threads, and sometimes for other science topics

Evolution threads I can follow, but this stuff just makes my brain hurt.

[gitmo]

There is a force associated with the electromagnetic quantum vacuum: the Casimir force.

[MonroeDNA]

Exactly right.

If you throw a bseball backeards, you get moved forwards. Conservation of mommentum.

If the baseball hits a trampoline, and comes back and hits you, you move forward sitll, though the trampoline moves backwards.

Rockets fly in space (vacuum) because they throw things backwards (hot particles). Nothing to push against, but nothing needs to.

"For every reaction, there is an equal, but opposite reaction." Jet engines, rocket engines, and propellers work, giving a nod to this statement.

[MonroeDNA]

The trampoline bounce-back thing is analogous to blasting of from a concrete pad, or,"pushing" against water, or anything.

[Semper911]

What is the problem, aside from earth should be lowercase?

Earth is properly capitalized. It is a proper noun -- the name of the planet.

I was lost in the second paragraph of the article, but the posts have been a great read. It has been a fascinating debate on the finer points of physics.

I love this place.

[MonroeDNA]

Fluid dynamics deals in presure graduations, as in the presure in front of the propeller, vs. the presure behind the propeller. It works well, but it is a simplification.

Momentum is everything, and if the propeller was stationary, and yet managed to expell water, the boat would move forward.

Forward pressure just fights the movement. When they are equal, the boat cannot move. Vacuum is the absence of opposing force; in this case, forward pressure.

But what do I know? I'm just an electrical engineer.

[RightWhale]

It is a proper noun -- the name of the planet.

Earth:
It is lowercase unless at the beginning of a sentence. That would be according to Webster's. Moon is also lowercase, as is sun, universe, world. For proper names we usually resort to Latin and use Terra, Luna, Sol, etc., and then the names are considered proper names and capitalized. It may be used with or without the definite article, as in earth, or the earth. It is sometimes capitalized anyway, depending on whether your English teacher insists.

[Toskrin]

The point seems pretty clear to me. On Earth, travel is usually performed by applying a mechanical force against an external mass - car wheels against the road, a propeller against air against air or water, your foot against the ground. All the travelling object needs is an appropriate form of mechanical linkage to push against the external mass.

In space, there's no external mass, so carrying propellant is the only way to get around.

What space travellers want is a way to get around with a solar powered motor.